Calorimetric sensors are often employed to measure gas concentration of one gas or a combination of gases. Certain prior art calorimetric sensors are constructed using noble metals as catalysts to determine concentrations of certain gases, or groups of gases. Noble metal catalysts promote oxidation of a combustible gases, or gases to be measured. Oxidation reactions generate heat, which causes a rise in temperature proximate an area of the reaction. A resulting increase in temperature can be sensed, and used to indicate a concentration of the combustible gas or gases under measure.
As a practical matter, catalysts are rarely selectively reactive to a particular gas molecule. As a result, gases other than the selected one, or selected group, of interest can cause interference during a measurement process. Therefore, the achievement of a reasonable degree of gas molecule selectivity is a major technical hurdle to overcome in sensor design.
An example of gas interference can be seen in application of a hydrocarbon sensor located in an exhaust gas plenum of a vehicle. Hydrocarbon sensors are required to measure concentration of non-methane (or heavy) hydrocarbons in internal combustion engine exhaust gas streams in the presence of other combustible gases.
The need for employing hydrocarbon sensors in vehicular applications, particularly automotive vehicles, is driven by environmental emissions legislation that is regularly being changed to require lower and lower emissions of pollutants from vehicles--hydrocarbons included. In particular, the California Air Resources Board (CARB) is leading an effort with their LEV (Low Emission Vehicle) and ULEV (Ultra Low Emission Vehicle) standards and the accompanying OBDII (On-Board Diagnostics II) requirement. For a LEV, OBDII requires detection of changes in catalytic converter non-methane hydrocarbon conversion efficiency of the order of 1-2%. This corresponds to a requirement to detect changes in emitted non-methane hydrocarbons of tens of PPM (parts per million).
Internal combustion engines exhaust gases including hydrogen (H.sub.2), carbon monoxide (CO), methane (CH.sub.4), carbon dioxide (CO.sub.2), non-methane hydrocarbons (CH.sub.x), nitric oxide (NO), and water vapor (H.sub.2 O). Sensors that are currently available cannot selectively detect non-methane hydrocarbons (CH.sub.x) in these engine exhaust gas streams. In hydrocarbon sensors using noble metals as a catalyst, it is very difficult to eliminate hydrogen (H.sub.2) interference, when other combustible gases such as non-methane hydrocarbons have to be measured selectively.
What is needed is an improved non-methane hydrocarbon sensor that is less sensitive to interference from combustible gasses in the exhaust gas stream, other than non-methane hydrocarbons, particularly hydrogen (H.sub.2)